Device and method for the simultaneous reading of passive inductive transponders

ABSTRACT

The present invention relates to a transponder reading device for reading out a plurality of inductive passive transponders ( 50 ) having a resonant circuit. A transmitter coil ( 62 ) is provided to create an alternating magnetic field having an operating frequency f B , in which the number of transponders ( 50 ) may be placed such that same are magnetically coupled to each other. A retuning means ( 56 ) is further provided which is arranged in the alternating magnetic field and has such a frequency-dependent impedance that a voltage induced in the resonant circuits of the transponders ( 50 ) by the alternating magnetic field has a maximum in the range of the operating frequency f B .

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to apparatus and methods enablingsimultaneous reading out of transponders, and in particular such methodsand apparatus enabling simultaneous reading out, or reading, of passiveinductive transponders arranged in a stack, so that the coils of thetransponders are closely magnetically coupled to each other.

[0003] 2. Description of Prior Art

[0004] With reference to FIG. 3, a conventional transponder system willbe described below which consists of a base station 10 and a transponder12, depicted schematically in FIG. 3. In the simplified representation,the base station 10 includes a power source 14, a resistor 16 seriallyconnected to the power source, and a capacitor 18 connected in parallelto the serious connection consisting of the power source 14 and theresistor 16. Connected in parallel to the capacitor 18 is a transmittercoil 20. Along with the capacitor 18, the coil 20 thus represents aparallel resonant circuit loaded, in the simplified example, by theresistor 16 and the power source 14, and in reality by the power outputstage.

[0005] The transponder 12 includes a transponder coil 22 and a tuningcapacitor 24. The transponder coil 22 and the tuning capacitor 24 definea parallel resonant circuit. In addition, in the simplified schematicrepresentation of the transponder 12, a rectifier diode 26 is connectedbetween the parallel resonant circuit and a capacitor 28 for storingenergy. A variable load, consisting of a resistor 30 and a switch 32, isarranged in parallel with the capacitor 28. Further, a transponder ASIC34 (ASIC=application-specific integrated circuit) is provided which isconnected to the switch 32 in the manner represented.

[0006] The base station 10 inductively supplies the transponder 12 withenergy 36, whereas the transponder returns transponder data 38 by meansof the load modulation method. Here, the transponder is a passivetransponder not requiring any additional source of energy.

[0007] In the base station 10, a current in the transmitter coil 20generates an alternating magnetic field 40 at an operating frequencyf_(B). The transponder coil 22 of the transponder 12 is arranged in thenear-field region of the transmitter coil 20, so that the alternatingmagnetic field 40 impinges upon the transponder coil 22 and induces anelectrical voltage there. The voltage induced serves the transponderASIC 34 as an operating voltage obtained via the rectifier 26 and thecapacitor 28.

[0008] To send data, the transponder ASIC 34, controlled by means of adata signal 42, switches the resistor 30 to be parallel with thecapacitor 28 via the switch 32, i.e. the transponder ASIC 34 switches onthe variable load. This leads to an increase in the power consumption ofthe transponder. Due to this power consumption of the transponder, thefield generated by the transmitter coil 20 is weakened. As aconsequence, the voltage applied at the resistor which exists betweenthe transmitter coil and the power output stage and is shownschematically as a resistor 16 in FIG. 3, increases in the base station10. These voltage changes in the base station 10 may be detected toreconstruct the data signal 42 sent by the transponder 12, such a methodbeing referred to as load modulation.

[0009] In a transponder system as has been explained above withreference to FIG. 3, the voltage induced in the transponder coil 22 mustbe higher than the operating voltage of the transponder ASIC. To enablethis, the tuning capacitor 24 is connected in parallel with thetransponder coil 22, the tuning capacitor forming a resonant circuitwith the transponder coil 22. In the transponder system shown, theresonant frequency of the transponder resonant circuit is the operatingfrequency f_(B), so that the voltage induced has a local maximum at theoperating frequency f_(B) depending on the frequency.

[0010] If several transponders exist in the field of the transmittercoil of a base station, and if these transponders are closelymagnetically coupled to each other, the transponders mutually influenceeach other. In addition to affecting the quality of the respectivetransponder resonant circuits, this influence also relates to theresonant frequency of the transponder resonant circuits, so that same nolonger matches the operating frequency. So far it has not been possibleto power and read out a stack of passive inductive transponders whichare arranged, in relation to each other, such that the coils of same areclosely magnetically coupled.

SUMMARY OF THE INVENTION

[0011] The present invention is based on the object of providingapparatus and methods enabling transponders arranged in a stack to bepowered and read out.

[0012] In accordance with a first aspect, the present invention providesa transponder reading device for reading out a plurality of inductivepassive transponders having a resonant circuit, the device comprising:

[0013] a transmitter coil for generating an alternating magnetic fieldhaving an operating frequency f_(B), in which the plurality oftransponders may be placed such that they are magnetically coupled toeach other; and

[0014] a retuning means arranged in the alternating magnetic field andhaving such a frequency-dependent impedance that a voltage induced inthe resonant circuits of the transponders by the alternating magneticfield has a maximum in the range of the operating frequency f_(B).

[0015] In accordance with a further aspect, the present inventionprovides a method for reading out a plurality of inductive passivetransponders having a resonant circuit, the method comprising:

[0016] generating an alternating magnetic field at/with/having anoperating frequency f_(B);

[0017] placing the plurality of transponders in the alternating magneticfield such that the transponders are magnetically coupled to each other;

[0018] placing a retuning means in the alternating magnetic field, theretuning means having such a frequency-dependent impedance that avoltage induced in the resonant circuits of the transponders by thealternating magnetic field has a maximum in the range of the operatingfrequency f_(B); and

[0019] receiving data created by the transponders in response to thevoltage induced.

[0020] The present invention is first of all based on the findings that,if several transponders are arranged in the field of the transmittercoil and if same are closely magnetically coupled to each other, thecircuit elements at the terminals of each transponder coil are mapped atthe terminals of every other transponder coil. This leads to anelectrical network containing coupled coils. This network is no longer asimple resonant circuit. The voltage induced no longer takes on amaximum at the operating frequency and is therefore no longer highenough to power the transponder ASIC.

[0021] Instead, in a plurality of transponders arranged in closemagnetic coupling to each other in the alternating magnetic field of thetransmitter coil, a maximum of the voltage or the current, respectively,induced in the transponder coils is caused by each transponder coil,i.e. by each transponder oscillating circuit. These maximums occur atvarious frequencies, the frequency positions of all maximums changing assoon as a further transponder is added in the alternating magneticfield. If the transmitter coil is also part of an oscillating circuit,as in the example described above with reference to FIG. 3, a maximum ofthe voltage induced in the transponder coils is caused also by theoscillating circuit of the transmitter coil.

[0022] As has been mentioned above, the various oscillating circuitsinfluence each other, so that, depending on the number and the nature ofthe transponder coils stacked, the voltage maximums have frequencypositions deviating from the resonant frequencies to which theindividual transponder resonant circuits are tuned. The transpondersthus cannot be read out. Since the transponders have fixed devicevariables, for example a transponder coil inductance of 5 μH and atuning capacitance of 2 pF, it is not possible to access the resonantfrequency of the resonant circuits of the transponders. In addition,there are statutory specifications for the operating frequency f_(B), sothat same cannot be changed either, a common operating frequency fortransponder systems being 13.56 MHz.

[0023] In accordance with the invention, a retuning means is thereforeintroduced into the near field of the transmitter coil such that theamplitudes of the voltages induced in the transponder coils, and thecurrents induced which are proportional to these voltages, take on alocal maximum at the operating frequency f_(B), i.e. the frequency ofthe transmitting voltage, and are sufficiently high to enable poweringof the transponder ASIC. In accordance with the invention, the retuningmeans introduced into the proximity of the transponder stack and thusinto the near field of the transmitter coil preferably comprises one orseveral coils magnetically coupling to the transponder coils. Forcreating a resonant circuit, a capacitor may be connected to the coil ofthe retuning means. Additional network elements, for example resistorsor further coils, may also be connected to the coil.

[0024] Due to the retuning means introduced into the alternatingmagnetic field of the base station, the voltages and/or currents inducedin the transponders exhibit an additional maximum, the frequencyposition of which may be set by a corresponding selection of thefrequency-dependent impedance of the retuning means. Thus, the frequencyposition of this additional maximum may be set to coincide with theoperating frequency f_(B), so that all transponders may be read out atthe operating frequency. By the introduction of the retuning means intothe alternating magnetic field, the other maximums of the voltage or thecurrent, respectively, induced in the transponder coil will be shifted,it being possible to shift one of these maximums to the operatingfrequency f_(B). This additional maximum is used for a small number oftransponders in the transponder stack, whereas a shifted maximum is usedfor a large number of transponders in the transponder stack.

[0025] In accordance with preferred embodiments of the presentinvention, the frequency-dependent impedance of the retuning means ispreferably variable, provisions preferably being made for means forvarying the frequency-dependent impedance until all transponders in atransponder stack respond. It is thus possible to ensure a safe readoutof all transponders in a transponder stack without knowing the number oftransponders in the stack.

[0026] In accordance with a further aspect, the present inventionfurther provides a transponder means with a resonant circuit consistingof a transponder coil and a tuning capacitor, the resonant circuit beingtuned such that a lower minimum cut-off frequency is in the range of apreset operating frequency f_(B) of an alternating magnetic field, thelower minimum cut-off frequency f_(u,min) being the frequency approachedby the lowest-frequency maximum of the voltage induced in thetransponder resonant circuit by the alternating magnetic field of theoperating frequency f_(B) if a theoretically unlimited number oftransponder means are arranged in the alternating magnetic field.

[0027] In accordance with the above-mentioned third aspect, the presentinvention is based on the findings that the frequency, at which theabsolute maximum of the voltage or the current, respectively, inducedoccurs, drops and approaches a lower limit, i.e. a lower minimum cut-offfrequency, as the number of transponders in the transponder stackincreases. If the transponder resonant circuits in such a transponderstack are dimensioned and/or tuned such this lower minimum cut-offfrequency occurs at the operating frequency f_(B), it is possible toread out the plurality of transponder means without requiring anyretuning means. In a different approach, one of the plurality oftransponder means may be considered the retuning means in such a case.

[0028] Further developments of the present invention are set forth inthe dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Preferred embodiments of the present invention will be explainedin more detail below with reference to the accompanying figures, inwhich:

[0030]FIG. 1 shows a schematic representation for explaining theprinciple on which the present invention is based;

[0031]FIG. 2 shows a schematic representation of an embodiment of thepresent invention; and

[0032]FIG. 3 shows a schematic simplified representation of atransponder system of the prior art.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0033] With reference to the schematic representations of FIGS. 1 and 2,a preferred embodiment of the present invention will be explained inmore detail below.

[0034]FIG. 1 shows a schematic arrangement of four transponders 50arranged in a stack such that the coils (not shown in FIG. 1) of thetransponders are closely magnetically coupled to each other. FIG. 1further shows a base station 52 which may be a transponder readingdevice or a transponder reading/writing means, as was explained abovewith reference to FIG. 3. The present invention will be described belowwith reference to a reading device in each case, those skilled in theart recognizing, however, that the base station may comprise anappropriate means for also enabling writing onto transponders arrangedin the alternating magnetic field of same.

[0035] The transponders 50, which, for example, have the structuredescribed above with reference to FIG. 3, are arranged in the near fieldof the transmitter coil (not shown in FIG. 1) of the reading device 52.

[0036] The transmitter coil of the reading device 52 creates analternating magnetic field at an operating frequency f_(B).

[0037] The resonant circuits of the transponders 50 are tuned to aresonant frequency via respective tuning capacitors 54. In commontransponder systems serving to read out an individual transponder, theresonant frequency of the resonant circuit of the transponder is theoperating frequency of the reading device. In the arrangement shown inFIG. 1, however, the resonant frequencies of the resonant circuits ofthe individual transponders 50 affect each other, so that same may notbe read out at the operating frequency f_(B) of the reading device.

[0038] In accordance with the present invention, a retuning means 56 isthus provided in the alternating magnetic field of the reading device52. In the preferred embodiment of the present invention, the retuningmeans consists of an oscillating circuit, the coil of which (not shownin FIG. 1) is magnetically coupled to those of the transponders. Themagnetic coupling of the coils of the transponders 50, the readingdevice 52 and the retuning means 56 is indicated by the symbol for amagnetic transmitter {overscore (M)}. In preferred embodiments of thepresent invention, the oscillating circuit of the retuning means 56 isdetunable as is shown by an adjustable capacitor 58 in FIG. 1.Alternatively, the inductance of the coil of the retuning means 56, orboth variables, could be changed.

[0039] An additional maximum is produced in the currents and/or voltagesinduced in the transponder coils of the transponders 50 by the retuningmeans. In addition, the maximums produced by each of the transpondercoils are shifted with regard to their frequency positions. Selectingthe frequency-dependent impedance of the retuning means 56 in a suitablemanner can thereby cause one of the voltage or current maximums,respectively, induced in the transponder coils to be at the operatingfrequency f_(B), so that all transponders of a transponder stack may beread out at the operating frequency f_(B) stipulated by law.

[0040]FIG. 2 shows a schematic representation of a possible arrangementof the coils towards each other in an inventive transponder readingdevice. FIG. 2 schematically shows the transponder coils 60 of fourtransponders arranged in a stack. In addition, FIG. 2 schematicallyshows a transmitter coil 62 of the reading device, and a coil 64 of theinventive retuning means. It shall be pointed out that FIG. 2 representsa schematic cross-sectional view of the respective coils. In thearrangement as shown in FIG. 2, the transponders of the transponderstack are arranged, with the transponder coils 60, in the alternatingmagnetic field of the transmitter coil 62 in such a manner that they areclosely magnetically coupled to each other and that they are furtherclosely magnetically coupled to the coil 64 of the retuning means.

[0041] The influence of the retuning means on the voltages or currents,respectively, induced in the transponder coils shall be set forthmathematically below. The voltages induced are proportional to thecurrents induced. In the consideration below, the currents areconsidered instead of the voltages, the electrical network, as is shown,for example, in FIG. 1, which contains the coupled coils of thetransponders, of the reading device and of the retuning means, beingexamined using mesh current analysis. The meshes are the transponderoscillating circuits and the electric circuit of the transmitter coil.The vector of the source voltages shall be referred to as {overscore(u)}₀. There is only one source voltage in the electric circuit of thetransmitter coil. The vector of the currents flowing through the coilsshall be referred to as {overscore (i)}. The following is true:

{overscore (u)} ₀ ={overscore (Z)} _(M) {overscore (i)}  (1)

[0042] In equation (1), {overscore (Z)}_(M) represents the meshimpedance matrix of the transponder oscillating circuits. On the maindiagonal, it contains the complex impedances of the transponderoscillating circuits, and on the secondary diagonals it contains termscaused by the mutual inductances. The currents thus are calculated asfollows:

{overscore (i)}={overscore (Z)} _(M) ⁻¹ {overscore (u)} ₀  (2)

[0043] In addition, the following applies: $\begin{matrix}{{\underset{\_}{\overset{\_}{Z}}}_{M}^{- 1} = \frac{\overset{\_}{A}}{\det \quad {\overset{\_}{\underset{\_}{Z}}}_{M}}} & (3)\end{matrix}$

[0044] The matrix {overscore (A)} is the matrix of the cofactors of{overscore (Z)}_(M).

[0045] It shall be assumed that the transponders are arranged in a stackand that their number is N. Depending on the frequency, the currents inthe N transponder coils take on N maximums even if the resonant circuitsof all transponders are set to the same resonant frequency. The reasonfor this is the mutual influence of the transponders due to the closemagnetic coupling of same. The solution to the equation

det={overscore (Z)}_(M)=0  (4)

[0046] determines where these N maximums lie. They spread across afrequency band in whose second third lies the resonant frequency f_(R)to which the individual transponders are tuned. The lower limit of theband, i.e. the lowest frequency at which a current maximum occurs in agiven arrangement, shall be referred to as f_(u). If f=f_(u), thecurrent induced takes on the absolute maximum. The amplitudes of themaximums drop as the frequency rises. If N is increased, the frequencyf_(u) drops but still approaches a lower limit f_(u,min).

[0047] As has already been set forth, the maximums of the currentinduced do not coincide with the operating frequency f_(B) due to thepreset transponder parameters as well as due to the set operatingfrequency f_(B). To enable powering of the transponders, however, theamplitudes of the currents induced must take on a local maximum at thefrequency of the transmitting voltage f_(B). It is exclusively thesolutions to equation (3), and thus {overscore (Z)}_(M), that determinethe frequency at which these currents take on a maximum. The position ofthe transponders in the stack has no influence here as long as there isclose magnetic coupling of same. Once the developer has specified thegeometry of the coils, the number of turns, the distances between thetransponder coils, the resonant frequency and the quality of thetransponder oscillating circuits, {overscore (Z)}_(M) now only dependson N, i.e. on the number of transponders, or transponder coils. Theinventors have therefore recognized that a system which can power alltransponders, which are arranged in any number N desired in a stack, ata frequency, i.e. the operating frequency f_(B), must have a variableinfluencing {overscore (Z)}_(M), this variable being independent of N.This influence variable may then be changed, depending on N, such thatone of the maximums to be found by solving equation (3) is f_(B) in eachcase.

[0048] {overscore (Z)}_(M) is obtained by means of mesh currentanalysis. The resonant circuits consisting of the transponder coils andthe capacitors and resistors connected therewith, and the switchingcircuit consisting of the transmitter coil and the voltage source may bespecified as the meshes. If {overscore (Z)}_(M) is to be altered,additional meshes must be added to the network. Such additional meshesare added by the inventive introduction of a retuning means into thealternating magnetic field in close magnetic coupling to the transpondercoils. That way, for each additional mesh, one row is added at thebottom and one column is added on the right-hand side in the matrix{overscore (Z)}_(M). Thus, equation (1) has other solutions. Theamplitudes of the currents induced in the transponder coils thus take onmaximums at other frequencies. By adjusting the frequency-dependentimpedance of the retuning means in a suitable manner, one of themaximums can thus be set to the operating frequency f_(B).

[0049] In the inventive introduction of the retuning means it becomesclear that an additional maximum of the amplitude of the currentsinduced is produced, which can be demonstrated using a computersimulation. It shall be pointed out at this point that the currentmaxima produced each occur in all transponder coils, which becomesreadily obvious from looking at equations 1 to 4. In addition toproducing the additional maximum, the introduction of the retuning meansalso leads to a shift of the current maximums produced by the individualtransponder resonant circuits themselves. For a small number oftransponders in the stack N, the maximum produced by the retuning meansis more pronounced at frequencies of f<f_(u) than at frequencies off>f_(u). Thus, the transponders are preferably tuned such that, with asmall number of transponders in the stack N, f_(u)>f_(B). With a smallnumber of transponders, the impedance of the retuning means ispreferably set such that the frequency at which the additional currentmaximum occurs coincides with the operating frequency f_(B).

[0050] Alternatively, the transponders could be tuned to a differentresonant frequency. For example, for a small number of transponders,f_(u)≈f_(B) could apply. As soon as N grows, f_(u) becomes <f_(B). It ishighly unlikely that now one of the maximums produced by thetransponders is at the operating frequency f_(B). The retuning meanswould have to be used for producing a maximum of the frequency f withf=f_(B)>f_(u). The question of whether or not such a procedure isexpedient may be answered by means of the magnitude of the maximumbrought about by the retuning means.

[0051] If f_(u)>f_(B) applies, the retuning means produces a maximum ata frequency with f=f_(B), i.e. with f<f_(u). For f_(u)<f_(B), theyproduce a maximum of the frequency f=f_(B)>f_(u). As can be demonstratedin computer simulations, for example, the maximum for f<f_(u) is muchmore pronounced than for f>f_(u). For f<f_(u), smaller field strengthsof the transmitter coil are thus required to induce sufficient currentsin the transponder coils. It is therefore expedient to tune thetransponders such that f_(u)>f_(B) applies. In this case, benefit canfurther be drawn from the fact that f_(u) converges towards a lowerlimit f_(u,min), so that, if f_(u,min)=f_(B), the transponders areautomatically tuned correctly as soon as their number is large enough.

[0052] With a large number of transponders in the stack, the additionalmaximum is less pronounced, so that it may be preferred, with a largenumber of transponders in the transponder stack, that a shift of acurrent maximum to the operating frequency f_(B), the shift being causedby one of the transponder resonant circuits, is caused by the retuningmeans.

[0053] As has already been noted above, without the tuning means, thefrequency f_(u), at which the first current maximum occurs, approaches alower limit f_(u,min). If the number of transponders present in thetransponder stack is large enough for the first current maximum to occurat the minimum cut-off frequency f_(u,min), so that even theintroduction of further transponders into the transponder stack does notlead to a further drop in the frequency at which this maximum occurs, itis possible to read out all transponders without using a tuning means bytuning the transponders, i.e. the resonant circuits of same, such thatf_(u,min)=f_(B). If there is at least such a number of transponderspresent in the field of the transmitter coil that the lower cut-offfrequency f_(u,min) is reached, the amplitude of the current induced inthe coils has a maximum at f_(B), so that the transponders may bepowered and thus read out.

[0054] What is understood by a large or small number of transpondersresults from the minimum operating voltage of the transponder ASICs, thequality of the transponder oscillating circuits, and the strength of themagnetic field of the transmitter coil, as shall be explained below.

[0055] In the range around the frequency of the absolute maximum f_(u),a frequency band may be defined so that the transponders may operate assoon as f_(B) is within this band. The voltage induced is larger therethan any voltage preset by the operating voltage of the ASICs. The bandbecomes broader if the ratio of the voltage induced in the transpondercoils to the minimum operating voltage of the ASICs increases. Thevoltage induced is proportional to the magnetic field strength of thefield of the transmitter coil, so that the width of the band increasesas the field strength increases. There are two ways in which the qualityinfluences the width of the band. An increasing quality causes thevoltage induced to increase. At the same time, however, the so-called 3dB bandwidth associated with the maximum becomes smaller. Along withsame, the bandwidth of the band in which the operating frequency f_(B)must lie for the transponder to be powered decreases. Therefore, optimumquality is the consequence of a compromise.

[0056] Like f_(u), the frequency band shifts towards smaller frequenciesas the number N of transponders increases. The number of transponders isconsidered large if the operating frequency f_(B) is within thefrequency band around f_(u), so that the transponders may be operatedwithout any retuning means. This approach is based on the preconditionthat the transponders are tuned such that the lower limit f_(u,min) ofthe frequency of the first maximum fu equals the operating frequencyf_(B).

[0057] As has been explained above, in accordance with the invention, anoscillating circuit with an adjustable resonant frequency, by changingthe frequency-dependent impedance of same, is preferably used as theretuning means. By changing the capacitance of the capacitor and/or theinductance of the coil in the retuning means, provisions can thus bemade for a maximum of the amplitudes of the currents to form in thetransponder stack at f=f_(B). In order to enable a reading out of allcoils with an unknown number of transponder coils, cycles for varyingthe inductance and/or the capacitance of the retuning means may bepassed, for example, until all transponders respond. The fact that alltransponders respond may readily be detected in the reading device byobserving the power drawn from the alternating magnetic field. If alltransponders respond simultaneously, time division multiplex protocolsmay be used to evaluate the responses of the individual transponders,for example by associating a time slot with each transponder.

[0058] If there is a fixed number of transponders in the transponderstack, and if all other parameters are constant, such as, for example,coil geometries, numbers of turns, and distances between the transpondercoils, resonant frequency and the quality of the transponder oscillatingcircuits, etc., it is sufficient to once determine thefrequency-dependent impedance of the retuning means required to enable areadout of transponders with this fixed number of same.

[0059] The quality of the transponder resonant circuits and the fieldstrength of the magnetic field of the transmitter coil are preferablyselected in view of the statutory provision and the current consumptionas well as the minimum operating voltages of the transponders, such thatthe transponders are as insensitive as possible to mistuning. However,the broader the resonance spectrum of the transponder oscillatingcircuit becomes in order to provide insensitivity to mistuning, the moretransmitting power is required. However, since the transmitting powermust remain within the statutory norms, the resonance spectra of thetransponder oscillating circuits must be kept accordingly narrow.

[0060] The coil of the retuning means is preferably dimensioned suchthat it is coupled to the transponder coils in such a manner thatscattering is minimal. In the ideal case, which cannot be achieved, thecapacitance is to map itself directly at the terminals of thetransponder coils via the turns ratio, as with a scatter-freetransformer. Then the transponders would be mistuned towards a smallerresonant frequency, and the frequency band in which the maximums occurwould immediately shift towards smaller frequencies, so that with alimited number of transponders present in the field of the transmittercoil, the frequency at which the absolute maximum occurs, i.e. theabove-described frequency f_(u), may be shifted towards the operatingfrequency f_(B).

[0061] In summary, it can be stated that it is not possible tosimultaneously read out a plurality of transponders until a retuningdevice in accordance with the present invention has been introduced. Inthis context it shall be pointed out that the retuning means need notnecessarily be constituted by an oscillating circuit to this end. Eachelement having a frequency-dependent impedance may rather serve as theretuning means, this frequency-dependent impedance preferably beingvariable to enable, as has been described above, an unknown number oftransponders to be read out by varying the frequency-dependent impedanceaccordingly until all transponders respond. Here, thefrequency-dependent impedance may simply be formed by a short-circuitedcoil or by a coil connected to further network elements, for examplecapacitors, resistors, or coils. Here, the properties of the coilsand/or of the network elements may be varied depending on the number andnature of the transponder coils stacked to enable an unknown number oftransponders to be read out.

[0062] Needless to say, several coils and/or several oscillatingcircuits may be introduced as the retuning means into the alternatingmagnetic field of the transmitter coil and thus into a magnetic couplingto the transponder coils. In addition, several retuning means withdiffering frequency-dependent impedances may be introduced one by oneinto the field of the transmitter coil as retuning devices, thediffering frequency-dependent impedances being stipulated for differingnumbers of transponders, respectively, so that, if a correspondingnumber of transponders are present in the stack, reading out may beeffected by means of the retuning means associated.

What is claimed is:
 1. Transponder reading device for reading out a plurality of inductive passive transponders having a resonant circuit, the device comprising: a transmitter coil for generating an alternating magnetic field having an operating frequency f_(B), in which the plurality of transponders may be placed such that they are magnetically coupled to each other; and a retuning circuit arranged in the alternating magnetic field and having such a frequency-dependent impedance that a voltage induced in the resonant circuits of the transponders by the alternating magnetic field has a maximum in the range of the operating frequency f_(B).
 2. Transponder reading device as claimed in claim 1, wherein the retuning circuit comprises a coil.
 3. Transponder reading device as claimed in claim 2, wherein the retuning circuit further comprises at least one further network element which is connected to the coil and is selected from the group consisting of capacitors, resistors, and coils.
 4. Transponder reading device as claimed in claim 1, wherein the retuning circuit comprises a resonant circuit.
 5. Transponder reading device as claimed in claim 1, wherein the frequency-dependent impedance of the retuning circuit is variable.
 6. Transponder reading device as claimed in claim 5, wherein the resonant circuit of the retuning circuit comprises a coil and a capacitor, the inductance of the coil and/or the capacitance of the capacitor of the resonant circuit of the retuning circuit being variable.
 7. Transponder reading device as claimed in claim 5, which further comprises a controller for varying the impedance of the retuning circuit until a voltage induced in the resonant circuits of the transponders by the alternating magnetic field has a maximum in the range of the operating frequency f_(B).
 8. Method for reading out a plurality of inductive passive transponders having a resonant circuit, the method comprising: generating an alternating magnetic field having an operating frequency f_(B); placing the plurality of transponders in the alternating magnetic field such that the transponders are magnetically coupled to each other; placing a retuning circuit in the alternating magnetic field, the retuning circuit having such a frequency-dependent impedance that a voltage induced in the resonant circuits of the transponders by the alternating magnetic field has a maximum in the range of the operating frequency f_(B); and receiving data created by the transponders in response to the voltage induced.
 9. Method as claimed in claim 8, wherein the step of placing the retuning circuit includes the step of varying the impedance of the retuning circuit until the voltage induced in the resonant circuits of the transponders by the alternating magnetic field has a maximum in the range of the operating frequency f_(B).
 10. Transponder having a resonant circuit consisting of a transponder coil and a tuning capacitor, the resonant circuit being tuned such that a lower minimum cut-off frequency f_(u,min) is in the range of a preset operating frequency f_(B) of an alternating magnetic field, the lower minimum cut-off frequency f_(u,min) being the frequency approached by the lowest-frequency maximum of the voltage induced in the transponder resonant circuit by the alternating magnetic field of the operating frequency f_(B) if a theoretically unlimited number of transponders are arranged in the alternating magnetic field. 